The present invention relates to a manufacturing method of capacitor composed of a conductive polymer, having a small size and a large capacitance.
As electronic appliances are digitized recently, there is an increasing demand for capacitors having a small size and a large capacitance, and having a low impedance in a high frequency region.
Hitherto, using a conductive polymer having a high electric conductivity as the electrolyte for the capacitor, many capacitors having a low impedance in a high frequency region have been proposed.
It is proposed to compose a capacitor having a constitution in which a conductive polymer containing a skeletal structure in the repeating units of 3,4-ethylenedioxythiophene and p-toluene sulfonic acid anion as dopant is formed on aluminum having a dielectric film by chemical polymerization. Japanese Laid-open Patent No. 2-15611 proposes a manufacturing method of capacitor comprising a step of applying a solution of dissolving 3,4-ethylenedioxythiophene monomer and oxidant in a solvent, on an oxidized aluminum electrode, a step of removing the solvent at room temperature or by heating to form a conductive polymer layer by chemical polymerization reaction, and a step of washing away excessive oxidant from the conductive polymer layer by using water, and finally drying.
A report in collected papers at the 58th general meeting of Japan Society of Electrochemistry, pp. 251-252 (1991), presents a manufacturing method of film capacitor having a large capacitance, comprising a step of forming a dielectric made of electrodeposition polyimide thin film on an etched aluminum foil, and a step of forming an electrode by forming conductive polymer layers sequentially by chemical polymerization and electrolytic polymerization.
Japanese Laid-open Patent No. 9-293639 proposes a solid electrolytic capacitor having a constitution in which a capacitor element having an anode foil and a cathode foil wound through a separator made of glass paper is impregnated with a mixed solution containing 3,4-ethylenedioxythiophene and oxidant, and polyethylenedioxythiophene produced by polymerization reaction of monomer in the mixed solution permeating in the separator is held by the separator as an electrolyte layer.
Using ferric p-toluene sulfonate as oxidant, and ethylene glycol as solvent, a capacitor element impregnated with a mixed solution containing them is let stand at temperature of 25 to 100xc2x0 C., and a conductive polymer layer composed of polyethylenedioxythiophene is generated by polymerization reaction. Consequently, the generated conductive polymer layer is washed by using water or organic solvent, and dried. Such manufacturing method of capacitor is disclosed.
Japanese Laid-open Patent No. 6-325984 proposes a manufacturing method of impregnating a capacitor element composed of tantalum sintered metal with a cooled mixed solution containing pyrrole monomer and oxidant, and then polymerizing in the air at 25xc2x0 C.
However, in the case of the method of impregnating a capacitor element made of a sintered metal forming a dielectric layer with a cooled mixed solution containing polymerizable monomer and oxidant, and polymerizing at temperature less than the boiling point of the solvent, the solvent is evaporated from the voids of the surface of the sintered metal. Since the mixed solution is supplied more or less in the surface voids from the inside, the conductive polymer layer formed by polymerization reaction is collected near the surface. As the process of impregnation with mixed solution, polymerization and washing is repeated by few times (extremely once), the surface voids are clogged.
When surface voids are clogged, the following problems occur. If attempted to impregnate with the mixed solution, the mixed solution does not permeate into the inner part of the capacitor element. As the process is repeated more times, the conductive polymer film cannot be formed densely inside the capacitor element. Excessive oxidant cannot be removed by washing after forming the conductive polymer layer. Due to these problems, high capacitance attainment ratio and excellent capacitor characteristic cannot be obtained.
Or, in the case of polymerization reaction at temperature of less than the boiling point of the solvent, after impregnating a capacitor element having an anode foil and a cathode foil wound cylindrically through a separator with a cooled mixed solvent, the solvent is evaporated from open ends at both sides of the cylindrical form. Since the mixed solution is supplied more or less in the end portions from the inside, the conductive polymer layer formed by polymerization reaction is collected near the end surface. As the process of impregnation with mixed solution, polymerization and washing is repeated by few times (extremely once), the end surface is clogged. When the end is clogged, the following problems occur. If attempted to impregnate with the mixed solution, the mixed solution does not permeate into the inner part of the capacitor element. As the process is repeated more times, the conductive polymer film cannot be formed densely inside the capacitor element. Excessive oxidant cannot be removed by washing after forming the conductive polymer layer. Due to these problems, high capacitance attainment ratio and excellent capacitor characteristic cannot be obtained.
Besides, when the conductive polymer layer is used as electrolyte, a high electric strength property cannot be obtained.
Moreover, in order to improve the stability at high temperature and high humidity, it is required to remove the excessive oxidant in the conductive polymer layer by washing. After applying or impregnating with the mixed solution in the capacitor element, when the conductive polymer layer is formed by polymerization reaction at temperature of less than the boiling point of the solvent, the conductive polymer layer is likely to be swollen and peeled off at the time of washing by using ethanol or other organic solvent. If washed by using deionized water without using organic solvent, excessive oxidant cannot be removed. Accordingly, when exposed to high temperature and high humidity, decrease or loss of capacitance, and increase of impedance occur, and the characteristics tend to deteriorate.
In the case of a wound capacitor element, when the conductive polymer layer is formed by polymerization reaction at temperature of less than the boiling point of the solvent, the excessive oxidant cannot be removed by sufficient washing same as in the case above. Accordingly, when exposed to high. temperature and high humidity, decrease or loss of capacitance, and increase of impedance occur, and the characteristics tend to deteriorate.
It is hence an object of the invention to obtain a capacitor having a high capacitance attainment ratio, an excellent capacitor characteristic, and an excellent stability at high temperature and high humidity.
A first aspect of the invention presents a manufacturing method of capacitor comprising:
a step of preparing a mixed solution containing polymerizable monomer, oxidant, and solvent,
a step of impregnating a capacitor element with the mixed solution,
a step of heating the capacitor element impregnated with the mixed solution quickly to a temperature above the boiling point of the solvent, and
a step of forming a conductive polymer layer on the capacitor element, by chemical polymerization reaction between the polymerizable monomer and the oxidant.
A second aspect of the invention presents a manufacturing method of capacitor comprising:
a step of preparing a mixed solution containing polymerizable monomer, oxidant, and solvent,
a step of impregnating at least one side of a dielectric layer with the mixed solution,
a step of heating the dielectric layer impregnated with the mixed solution quickly to a temperature above the boiling point of the solvent, and
a step of forming a conductive polymer layer on the one side of the dielectric layer, by chemical polymerization reaction between the polymerizable monomer and the oxidant.
Preferably, the step of preparing the mixed solution containing the polymerizable monomer, the oxidant and the solvent is a step of preparing a cooled mixed solution.
Preferably, the capacitor element has a dielectric layer formed on a sintered metal.
Preferably, the capacitor element has a constitution in which an anode foil forming a dielectric layer and a cathode foil are wound through a separator.
Preferably, it further comprises a step of impregnating the conductive polymer layer with an electrolyte.
Preferably, the polymerizable monomer contains 3,4-ethylenedioxythiophene.
Preferably, the anode foil and the cathode foil have multiple penetration holes.
In the first constitution above, by heating quickly to exceed the boiling point of the solvent after impregnating the capacitor element with the mixed solution, the solvent is evaporated, the volume expands, and the solvent ejects from the voids of the sintered metal surface. Therefore, if the conductive polymer layer generated by polymerization reaction gathers near the surface of the sintered metal to block the voids temporarily, the conductive polymer layer generated provisionally is torn away by the ejecting force of the solvent. As a result, clogging of the voids of the sintered metal surface by conductive polymer film is prevented. Of if the conductive polymer layer generated by polymerization reaction gathers near the end surface of the wound capacitor element to block the end surface temporarily, the conductive polymer layer is torn away by the ejecting force of the solvent, and clogging of the end surface by conductive polymer film is prevented. Accordingly, by repetition of the process of impregnation with mixed solution, polymerization and washing, the conductive polymer layer can be formed densely in the capacitor element. By sufficient washing after forming the conductive polymer film, excessive oxidant can be removed. As a result, a capacitor having a high capacitance attainment ratio, an excellent dissipation factor, and a superior impedance characteristic is obtained.
Further, by impregnating the conductive polymer layer with electrolyte after forming the conductive polymer layer densely inside the capacitor element, the restoration capacity of defects of the dielectric layer can be enhanced. As a result, a capacitor having an excellent dielectric strength property is obtained.